SPC Products

The Storm Prediction Center (SPC) is part of the National Weather Service (NWS) and the National Centers
for Environmental Prediction (NCEP). Our mission is to provide timely and accurate forecasts and watches for
severe thunderstorms and tornadoes over the contiguous United States. The SPC also monitors for hazardous winter
weather and fire weather events and issues specific products for those hazards. We use the most advanced technology
and scientific methods available to achieve this goal.

SPC issues Day 1, Day 2, and Day 3
Convective Outlooks that depict non-severe thunderstorm areas and severe thunderstorm threats across the contiguous United States,
along with a text narrative. The categorical forecast specifies the level of the overall severe weather threat via numbers (e.g., 5),
descriptive labeling (e.g., HIGH), and colors (e.g., magenta). The probabilistic forecast directly expresses the best estimate of a severe weather event
occurring within 25 miles of a point. The text narrative begins with a listing of severe thunderstorm risk areas by state and/or geographic region. This is followed
by a concise, plain-language summary of the type(s) of threat along with timing that is focused on the highest-risk areas. The rest of the outlook
text is written in scientific language for sophisticated users. This technical discussion usually includes a synopsis section to provide a general overview of the weather pattern,
emphasizing features that will influence the severe and general thunderstorm threats. Additional sections of the discussion are usually separated by geographic areas.
Within these individual geographic areas, the text offers meteorological reasoning and justification for the type of coverage and intensity attendant to the severe weather threat.

SPC also issues a Day 4-8 Severe Weather Outlook that similarly depicts severe thunderstorm threats across the contiguous United States and
contains a technical discussion.

The level of categorical risk in the Day 1-3 Convective Outlooks is derived from probability forecasts of tornadoes, damaging winds, and large hail
on Day 1, and a combined severe weather risk on Days 2 and 3.

TSTM (light green) - General or non-severe thunderstorms - Delineates, to the right of a line, where a 10% or greater probability of thunderstorms is forecast during the valid period.

1-MRGL (dark green) - Marginal risk - An area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity.

2-SLGT (yellow) - Slight risk - An area of organized severe storms, which is not widespread in coverage with varying levels of intensity.

4-MDT (red) - Moderate risk - An area where widespread severe weather with several tornadoes and/or numerous severe thunderstorms is likely, some of which should be intense.
This risk is usually reserved for days with several supercells producing intense tornadoes and/or very large hail, or an intense squall line with widespread damaging winds.

5-HIGH (magenta) - High risk - An area where a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived
derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is
reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events).

Each Day 1 Convective Outlook is valid from the start of issuance (except for the 0600 UTC issuance which begins at 1200 UTC that day) through 1200 UTC the following day
(except for the 0100 UTC issuance which is valid through that day).
Each Day 2 Convective Outlook covers the period from 1200 UTC the following day to 1200 UTC the day after that. For example, if today is Monday,
then the Day 2 Convective Outlook will cover the period of 1200 UTC Tuesday to 1200 UTC Wednesday.
The Day 3 Convective Outlook covers the period of 48 to 72 hours from 1200 UTC on the morning of product issuance.
The Day 4-8 Severe Weather Outlook covers the period of 72 to 192 hours from 1200 UTC on the morning of product issuance.

This link describes the meaning of "Z" in our product issuance and valid times.

Two probabilistic thresholds of 15% and 30% can be forecast. Highlighted areas are equivalent to 2-SLGT-yellow or 3-ENH-orange risks on the Day 1-3 Convective Outlooks.
On rare occasions, the outlook text will begin with a headline for the possibility of a severe weather outbreak.

If no 15% areas are forecast, one of the following phrases will be used:

PREDICTABILITY TOO LOW: Used to indicate severe storms may be possible based on some model scenarios. However, the location or occurrence of severe storms is in doubt due to
large spread in model guidance and/or minimal run-to-run continuity.

POTENTIAL TOO LOW: Used to indicate that 15% or greater severe probabilities appear highly unlikely on that day.

Forecasting rare events such as tornadoes and the occurrence of large hail and damaging wind gusts is a difficult process and one that contains a large amount of uncertainty.
It is important not to rigidly associate the type of risk area (e.g., 2-SLGT-yellow) with the severe potential for any given thunderstorm in the risk area.
That is, just because a 2-SLGT-yellow risk is forecast does not necessarily mean that the thunderstorms within the risk area will be slightly severe.
Sometimes, violent tornadoes occur in 2-SLGT-yellow, 3-ENH-orange, or 4-MDT-red risk areas as opposed to 5-HIGH-magenta.
The reason for this is the synoptic situation producing the violent tornadoes may be confined to a relatively small area or a conditional, uncertain situation.
Another 2-SLGT-yellow risk area may cover several states in which only one or two tornadoes are expected to develop. Some 2-SLGT-yellow situations won't involve a threat of
tornadoes or supercells, but sustained multicell storms with a threat for severe hail and wind damage.

SPC severe weather outlooks forecast events from organized convection (e.g., supercells, squall lines, and multicell thunderstorm complexes), most capable of
damage and injury from tornadoes, damaging winds, or large hail. Pulse-type thunderstorms, consisting primarily of solitary brief severe updrafts (often found in
environments with weak vertical wind shear) are not considered organized. Convection of this type, may not be included in a risk area, unless forecaster confidence
is high enough to draw a 1-MRGL-dark green risk. Since almost any thunderstorm can produce a brief severe weather event, it doesn't necessarily mean there is a conflict
when a severe thunderstorm warning is issued by a local NWS office outside of an SPC severe weather risk area.

In short, no two situations are alike, even within the same risk category. This is why a probabilistic forecast and text discussion accompanies the categorical outlook.
The probabilities used in the SPC Convective Outlooks are known as subjective probabilities. The forecaster makes their best estimate of the probability of an event occurring.
The probabilities that you see on the graphics represent the probability of one or more events occurring within 25 miles of a point during the outlook period. This definition
is used as the probability of severe weather at a given point is quite small. How many times have you experienced a tornado in your neighborhood? For most people, the answer
is never. Now think of how many times severe weather has occurred within 25 miles of your location. It's probably safe to say that you can think of some close-by severe weather
events. You should be able to imagine that the probability of having severe weather occur within such an area is much larger than the probability of having it occur specifically
within any one neighborhood.

How should you interpret probabilistic values? The smallest values represent areas where the most uncertainty exists and correspondingly where the smallest expected coverage of storm reports
exists. The higher the probabilities, the greater the perceived threat, and the greater the expected coverage of that hazard being forecast. The highest probabilities are generally
reserved for more significant severe weather events and are used infrequently, if at all, during the year. Another way of thinking of the values is related to climatology.
For example, let's assume that the SPC forecaster drew a 30% area for tornadoes which included northwest Texas and southwestern Oklahoma in mid-May. The ratio of the forecast to
climatology (30%/1.5%) yields a value of approximately 20. The SPC forecaster is stating they believe the risk of tornadoes in that region is 20 times larger than climatology.
By comparing the forecast probability to climatology, you can better determine the magnitude of the risk on a given day. See the SPC
Severe Weather Climatology page to find climatological values for where you live.

The most specific Convective Outlooks are those issued during the Day 1 period. Accordingly, SPC forecasters have the most information available to them to differentiate the
threats of the individual severe weather hazards. During this period, the SPC produces probabilistic outlooks for each primary severe weather hazard (tornadoes, damaging wind,
and large hail) separately. By producing separate forecasts for tornadoes, damaging wind, and large hail, the user is given substantially more information upon which to make decisions
than in the categorical outlook. Users who are sensitive to one particular threat (e.g., car dealers and large hail) can make more informed decisions.

Since many of the specific details of severe weather forecasting can only be determined hours ahead of time, rather than several days, the severe weather probabilities for the
Day 2 and Day 3 Convective Outlooks represent the probability of any severe weather hazard (large hail, damaging wind, or tornadoes) occurring. In addition, for tropical cyclones
(hurricanes, tropical storms, or depressions), the outlooks on Day 2 and Day 3 allow a 5% total severe probability to be a SLGT risk because they are specifically tornado-driven.

The points product for each outlook can be found at a link
at the bottom of the Day 1-3 Convective Outlooks. Standard aviation identifier location codes are used to delineate the risk areas on the Day 1-3 Convective Outlooks.
A list of many of these identifiers can be found via
UCAR.
When plotted with a line drawn between each point, the outlined area
forms a polygon.
The points may either fall exactly on top of the location identifiers
(i.e. DAL...SPS...GAG) or may be referenced from those points
(i.e. 20 NW FMY...10 E MIA) in which case the point would be xx number
of nautical miles in the given direction from that point.
The previous example would read 20 nautical miles northwest of
Ft. Myers FL to 10 nautical miles east of Miami.
The designator "...CONT..." is used to indicate that the risk area
goes to the U.S. border, then starts again at another location on the border.
For example, part of a risk area might say "MSP INL ...CONT... SSM".
This means the risk area goes from Minneapolis-Saint Paul to International Falls
then comes back in from the Canadian border at Sault Ste. Marie.

SPC issues Thunderstorm Outlooks that depict the probability of thunderstorms across the contiguous United States in 4 or 8 hour time periods.
The probabilistic forecast directly expresses the best estimate of a thunderstorm occurring within 12 miles of a point. The three probabilistic forecast thresholds are: 10, 40, and 70 percent.
The following table details the outlook issuance time and then the valid times for up to three forecast periods.

0600Z: 1200-1600Z, 1600-2000Z, 2000-0000Z

1300Z: 1600-2000Z, 2000-0000Z, 0000-0400Z

1700Z: 2000-0000Z, 0000-0400Z, 0400-1200Z

2100Z: 0000-0400Z, 0400-1200Z

0130Z: 0400-1200Z

Forecasts of thunderstorms are critical for the protection of life and property since every thunderstorm contains lightning, which is a potential killer. The enhanced temporal resolution
of the SPC Thunderstorm Outlooks aid NWS forecasters and partners in time-sensitive decisions related to thunderstorms.

The Public Severe Weather Outlook (PWO) is issued on Day 1 for all category 5-HIGH-magenta risks and 4-MDT-red risks
that are driven by tornadoes and/or damaging winds. This plain-language forecast is typically issued the morning of the event and is used to alert non-technical weather
users concerned with public safety of a potentially dangerous situation. If the probabilities support a PWO issuance on the 0600 UTC Day 1 Outlook, a PWO
would be issued by 1000 UTC and updated around 1700 UTC. If the probabilities first support a PWO issuance on the 1300 UTC Day 1 Outlook, the PWO would be issued around 1300 UTC
and updated around 1700 UTC. A PWO is also issued following the 2000 UTC or 0100 UTC Day 1 Convective Outlook when a 10 percent or greater probability of significant tornadoes is
expected to occur at night.

SPC issues Mesoscale Discussions (MDs or MCDs) that focus on severe thunderstorm potential over the continental U.S. for the next 6 hours with an emphasis
on the first 1-3 hours. SPC also issues MDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards, and freezing rain.
All MDs contain an areas affected line, concerning line, valid time, a paragraph for a summary, and a paragraph for a technical discussion, along with a graphical depiction of the highlighted
area.

Development and/or evolution of severe convection in relation to watch potential or within valid watches

Alerting users to an upcoming categorical upgrade in the Day 1 Convective Outlook

A categorical upgrade to the 0100 UTC Day 1 Convective Outlook, after the issuance of the 0600 UTC Day 1 Convective Outlook.

The concerning line of a severe weather MD will reference an ongoing watch (e.g., TORNADO WATCH 559), or provide a confidence level on the expectation of a watch (including the type of watch).

SEVERE POTENTIAL...WATCH UNLIKELY (5 or 20%)

SEVERE POTENTIAL...WATCH POSSIBLE (40 or 60%)

SEVERE POTENTIAL...WATCH LIKELY (80 or 95%)

SEVERE POTENTIAL...TORNADO WATCH LIKELY (80 or 95%)

SEVERE POTENTIAL...SEVERE THUNDERSTORM WATCH LIKELY (80 or 95%)

SEVERE POTENTIAL...WATCH NEEDED SOON (95%)

The first paragraph of the MD provides a plain-language summary that details the forecast severe threat (i.e., timing, coverage, intensity, and mode) and resultant SPC actions.
The second paragraph of the MD describes the significant mesoscale features and atmospheric processes which will likely result in the expected event.

Generally, a severe weather MD is written whenever insightful information concerning mesoscale convective developments can be conveyed. The MD is written concisely and in an understandable
style, with an emphasis on the mesoscale aspects of the situation. Severe weather MDs provide extra lead time on the severe weather development and allow you to begin gearing
up operations before a watch is issued. The SPC goal is to issue severe potential MDs 1-2 hours prior to the watch issuance.

Winter weather MDs focus on the meteorological processes expected to cause hazardous winter weather over the continental U.S. for the next 6 hours, with emphasis on the first 4 hours.
The MD provides short term forecast information on the what, when, where, and why of the impending weather hazard. In the first paragraph, the plain-language summary contains information
concerning expected snowfall/precipitation rates and timing. The second paragraph discussion states the mesoscale meteorological processes that can be diagnosed and forecast.

When conditions become favorable for organized severe thunderstorms or tornadoes to develop, the SPC issues a Severe Thunderstorm or Tornado Watch.
A tornado can occur in either type of watch, but Tornado Watches are issued when conditions are favorable for either multiple tornadoes or a single intense tornado. A Severe Thunderstorm
Watch is issued when there is a forecast of organized convection producing at least 6 total severe weather events. Watches encourage the general public to stay alert for changing
weather conditions and probable warnings. For emergency managers, storm spotters, and the broadcast media, watches provide valuable lead time to gear up operations and increase staffing.
The watch is defined by the counties within a watch that are collaborated with local NWS offices. Therefore, some counties in the watch may be outside the parallelogram
(which approximates the watch area), while some counties in the parallelogram may not be in the actual watch issuance.

A typical watch ranges in size from 20,000 to 40,000 square miles, though some are smaller and others larger, depending on the meteorological
situation. A typical watch duration is 6-8 hours, but it may be canceled, extended in time or space, or replaced as required. Watches are canceled and extended only at local NWS offices, in
consultation with SPC. A watch is not a warning, and should not be interpreted as a guarantee that there will be severe weather!

SPC's goal is to issue watches shortly prior to the development of severe thunderstorms and tornadoes. The goal is to issue the initial Severe Thunderstorm Watch of the day with a
lead time of at least 45 minutes. For a Tornado Watch, the lead time goal is 2 hours for the first tornado event and 1 hour to the first non-tornado severe weather event (hail or wind).
For all watches, the lead time should average around 1 hour for a Severe Thunderstorm Watch and 2 hours for a Tornado Watch.

Whenever there is a high confidence forecast of multiple intense tornadoes (rated EF2-EF5), SPC will highlight a watch with the
following "PDS" wording:

...THIS IS A PARTICULARLY DANGEROUS SITUATION...

The SPC goal is to have 3 out of every 4 PDS Tornado Watches verifying with multiple intense tornadoes.

Similar PDS wording is used in Severe Thunderstorm Watches when a well defined, large bow echo has developed and there is evidence of widespread damaging winds occurring
and the bow echo is moving at 55 mph or greater and downstream conditions suggest the bow echo will be maintained or intensify for the duration of the watch. A PDS
Severe Thunderstorm Watch can also be issued for the anticipation of widespread significant severe weather events (convective winds greater than 75 mph and/or hail greater than 2 inches in diameter).

Unfortunately, not all severe weather situations are clear cut. For example, severe weather may be expected if thunderstorms form,
but there may be doubt about whether storms will develop. In such cases, SPC may wait until storms actually develop before they issue a watch.
Sometimes warnings may precede a watch, especially when weaker severe storms develop before the greater severe threat is expected to occur.
If severe weather develops unexpectedly, but is expected to be short lived (last less than a couple of hours) or is only very isolated,
a watch probably will not be issued. Instead, the storms would be handled with warnings issued by a local NWS office.

The Public Watch graphic (available on the SPC web site) depicts the outline, type, and valid time of the watch. The text provides in plain language
the: type of watch, state(s) affected, valid time, primary hazard(s), areal approximation, precautionary/prepardness actions, replacement of other watches (if any),
a short weather discussion, and aviation information.

Since not all watches are created equal, the primary hazard language is driven by watch probabilities. Two probabilities for each severe hazard (tornadoes, damaging winds,
and hail) are included along with the probability of 6 or more severe events.

The Watch Outline Update (WOU) contains a listing of all counties in the watch. The local NWS offices will also issue a Watch County Notification (WCN)
message that lists the counties in the watch within their area of responsibility. Once the watch is issued, the local offices will issue WCN messages
to cancel or add counties, or to extend the watch expiration time. The WOU is updated at least hourly to incorporate the changes made in the WCNs.

In the Aviation Watch text, the storm top numbers are in hundreds of feet; so "500" is 50,000 feet.
The mean storm motion vector is the average expected motion of all the storms in the watch: The first 3 digits are direction the storms
will move from on a 360-degree compass; and the last two digits are the storm's expected forward (ground) speed in knots. So for a vector of "23045",
storms are forecast to move from 230 degrees (from the southwest) at 45 knots.

Watch Status Reports are issued at the bottom of each hour (between 20 and 40 minutes after), during the lifetime of each severe
thunderstorm and/or tornado watch. The first status report usually will not be issued until the watch has been in effect for an hour.
Each Watch Status Report uses the distance in statue miles relative to anchor points and to the right of a line for
delineating where the severe weather threat continues. The watch status lines are drawn from one edge of the watch parallelogram to the other edge.

The Watch Status Report rarely includes a discussion of the meteorology affecting the watch area, as this information will be contained
in a Mesoscale Discussion (MD). The exception may be with the final Watch Status Report, where a brief sentence sometimes will be included explaining why the watch
will be reissued or allowed to expire.

The Fire Weather Outlooks are intended to delineate areas of the continental U.S. where pre-existing fuel conditions, combined with forecast weather conditions during the
next 8 days, will result in a significant threat for the ignition and/or spread of wildfires. This product is designed for use in the NWS, as well as other federal,
state, and local government agencies.

Each outlook consists of a categorical forecast that graphically depicts fire weather risk areas across the continental United States, along with a text narrative.
Through various labels and colors on the graphic, the five types of Fire Weather Outlook risk areas are:

Extremely critical delineations are made when wind, relative humidity and temperatures significantly deviate from climatological normals, but can be made for
borderline weather conditions where exceptional drought exists.

SPC utilizes fuel dryness level grids produced by Geographic Area Coordination Centers (GACCs). Fuel dryness levels of dry or very dry are necessary for the issuance of any type
of SPC fire weather outlook area. When GACC dryness level grids are not available, SPC considers fuels to be dry where there is a National Fire Danger Rating System
Adjective Class Rating of at least High, or 100/1000-hr dead fuel moisture below 10 percent in the West, or 10-hr dead fuel moisture below 10 percent in the East, or at least severe drought.

If an area is being considered for a possible Critical area but there is low confidence, or the expected weather conditions will be just below the aforementioned criteria
for a Critical area, an Elevated area is highlighted. Where Critical areas are highlighted, an Elevated area will surround the Critical area. Guidelines for an Elevated area
are provided below:

Elevated for dry thunderstorms, or wind and relative humidity:

10-39% coverage (isolated) of cloud-to-ground lightning strikes with rainfall accumulation at or below 0.10" over dry fuels OR

A combination of sustained wind speeds of 15 mph or greater (10 mph Florida), relative humidity up to 5% above regional critical thresholds (see this graphic),
and temperatures at or above 45-55 degrees F (depending on the season), for a duration of 3 hours or more over dry fuels OR

Brief (duration of less than 3 hours) and/or localized exceedance of critical thresholds over dry fuels OR

The
Day 1 Fire Weather Outlook is issued twice a day: at 400 am (CST and CDT), with an update by 1700 UTC. This outlook covers the period from 1200 UTC today through
1200 UTC the following day (except for the update where the period begins at issuance time). Each outlook consists of a categorical forecast that graphically
depicts fire weather areas across the continental United States, along with a text narrative.

The outlook text begins with a listing of Critical and Extremely Critical areas by state and/or geographic region. For the initial issuance of the day, a synopsis provides
a brief discussion of the overall synoptic pattern, with an emphasis on weather systems that would affect potential fire weather areas. The rest of the outlook text is
usually separated by geographic areas. Within these individual geographic areas, a meteorological narrative emphasizes predominant forecast conditions such as sustained wind speeds,
minimum RH values, temperatures, and/or coverage of dry thunderstorms. Meteorological justification for the type of risk issued is also provided. For the outlook
update, the text emphasizes changes made to the previous outlook and/or observational updates.

The Day 2 Fire Weather Outlook
is issued twice a day: at 1000 UTC, with an update by 2000 UTC. This outlook covers the period from 1200 UTC the following day to 1200 UTC the day after that.
For example, if today is Monday then the Day 2 Fire Weather Outlook will cover the period 1200 UTC Tuesday to 1200 UTC Wednesday.
This outlook is similar to the Day 1 Fire Weather Outlook in terms of the graphic and text.

The Day 3-8 Fire Weather Outlook
is issued once a day at 2200 UTC. This outlook covers the period of 48 to 192 hours from 1200 UTC on the morning of product issuance.
This outlook consists of a categorical forecast graphic and text, similar to the Day 1 and Day 2. However, only Critical areas
(for both wind and relative humidity, as well as dry thunderstorms) are highlighted. In addition, the outlook text does not list the Critical areas by state and/or geographic region.

If no Critical areas are forecast, one of the following phrases will be used:

PREDICTABILITY TOO LOW: Used if a critical fire weather area is possible during the period, but it is too uncertain to delineate at the time of issuance due to state of fuels
and/or forecast weather conditions.

POTENTIAL TOO LOW: Used to indicate a very low threat for a critical fire weather area during the forecast period.

SPC automatically compiles preliminary local storm reports (LSRs) issued by local NWS offices. This data is available in multiple formats on this
page. For each 24 hour period beginning at 1200 UTC on the valid day, a graphic and listing of all continental U.S. preliminary severe weather reports
that SPC received is available. Since the logging process is automated, missing or improperly formatted reports from local NWS offices will not get into the database. Reports
may be reclassified as well. What's initially reported as a tornado today might be called thunderstorm wind damage a few days later after a survey is done to verify the cause
of the damage. The log often contains duplicate reports, especially of tornadoes when multiple sightings of the same tornado were sent in the local storm reports.

Reports are binned into three categories: tornado, hail, and damaging winds. The log is a rough listing of all reports received. The final list of
reports is found in the Storm Events Database, which is compiled by the National Climatic Data Center (NCDC) a few months
after final report lists are submitted by local NWS offices. There is no guarantee as to the accuracy of SPC rough log reports and should be regarded as
strictly preliminary.

The Monthly and Annual U.S. Tornado Summaries are updated by SPC at irregular intervals.
The Latest U.S. Tornado Statistics provides a monthly summary of tornadoes by month and contain data
for the last four years. PREL stands for preliminary, which should match the SPC rough log totals. ACT is the actual number from the
Storm Events Database; AV is average. ACT numbers include removal of any erroneous/duplicate reports or
added reports which were initially missed or misclassified. An "--" in a column means the data is not yet available.
There is also a column called "3YR AV" that gives the average number of tornadoes per month (based on the 3 years' data).

The NUMBER OF TORNADO DEATHS columns are the number of people killed by month for the years listed and the average killed (3 years) per month. The
KILLER TORNADOES columns represent killer tornado events for the current year and the 3-year average. A tornado is counted as a killer if one or more persons were killed.
If 100 people were killed by a single tornado it would be counted as one killer event. Multiple killer tornadoes on the same day are counted as separate events.

The Annual U.S. Killer Tornado Statistics provides a graphical map of killer tornadoes, while
Current Year Preliminary U.S. Killer Tornadoes provides a table listing of
killer tornadoes in chronological order. The DEATHS column is number of deaths in the whole tornado path, not just at the given location. Information for the killer
tornadoes list comes from Local Storm Reports (LSRs) and Public Information Statements (PNS) issued by local NWS offices, supplemented by news from internal NWS event
memos and Internet media accounts. Since killer tornado information, especially death counts, circumstances, and EF-scale, is often not complete until many days later,
these numbers are very preliminary and subject to change as more information arrives.